Steroid hormones regulate cell function by binding to cytoplasmic receptor proteins. These interact with acceptor sites in target cell nuclei. There they induce messenger RNAs for specific cellular proteins which are responsible for alterations in endocrine physiology. Progesterone has been difficult to study at the molecular level in mammalian systems. We have therefore characterized the progesterone-sensitive chick oviduct over the past 14 years and have shown it to be a representative model of steroid hormone action. The structure-function relationships of the chicken oviduct progesterone receptor will be studied in detail. This hormone binding protein has now been cloned in our laboratory and following expression in suitable vector-host systems will be able to be purified in amounts suitable for detailed protein structural analyses. The protein's subunit structure, DNA binding to the ovalbumin, ovomucoid, and hsp108 genes, hormone-binding mechanism, and effects on endocrine target cells will be studied in vitro. The hormone binding site will be analyzed finally by X-ray crystallography. The origins and relationships of sequence and secondary structure between subunits will be determined, together with detailed analysis of contracts between the two subunits in the intact complex. DNA interactions of subunit A will be defined precisely with respect to effects on DNA higher order structure. A cell-free oviduct transcription system has been developed for tests of effects of purified receptors in vitro using purified transcription factors from homologous oviduct cells. This work should afford us an opportunity to define precisely the molecular basis of hormone-mediated gene expression, resulting in an advance in our understanding of endocrine physiology, reproductive biology and steroid hormone action.